Memory module having memory devices containing internal device ID registers and method of initializing same

ABSTRACT

A method and apparatus for assigning identification values to memories. A master resets identifiers of a first memory and a second memory by sending a reset signal on a line that is coupled in a daisy-chained manner to the first and second memories and also coupled to the master. The master places a first identification value on a data bus coupled to the master and to the first and second memories. The first memory stores the first identification value on the data bus as an identifier for the first memory when the master sends a first storage signal to the first memory via the daisy-chained line. The master places a second identification value on the data bus. The second memory stores the second identification value on the data bus as an identifier for the second memory when the master sends a second storage signal to the second memory via the daisy-chained line.

Divisional of prior application Ser. No. 08/762,139 Dec. 9, 1996 U.S.Pat. No. 5,809,263, which is a continuation of Ser. No. 08/607,780 Feb.27, 1996 abandoned and a continuation of Ser. No. 08/222,646 Mar. 31,1994 U.S. Pat. No. 5,513,327 and a continuation of Ser. No. 07/954,945Sep. 30, 1992 U.S. Pat. No. 5,319,755 and a continuation of Ser. No.07/510,898 Apr. 18, 1990 abandoned.

FIELD OF THE INVENTION

An integrated circuit bus interface for computer and video systems isdescribed which allows high speed transfer of blocks of data,particularly to and from memory devices, with reduced power consumptionand increased system reliability. A new method of physicallyimplementing the bus architecture is also described.

BACKGROUND OF THE INVENTION

Semiconductor computer memories have traditionally been designed andstructured to use one memory device for each bit, or small group ofbits, of any individual computer word, where the word size is governedby the choice of computer. Typical word sizes range from 4 to 64 bits.Each memory device typically is connected in parallel to a series ofaddress lines and connected to one of a series of data lines. When thecomputer seeks to read from or write to a specific memory location, anaddress is put on the address lines and some or all of the memorydevices are activated using a separate device select line for eachneeded device. One or more devices may be connected to each data linebut typically only a small number of data lines are connected to asingle memory device. Thus data line 0 is connected to device(s) 0, dataline 1 is connected to device(s) 1, and so on. Data is thus accessed orprovided in parallel for each memory read or write operation. For thesystem to operate properly, every single memory bit in every memorydevice must operate dependably and correctly.

To understand the concept of the present invention, it is helpful toreview the architecture of conventional memory devices. Internal tonearly all types of memory devices (including the most widely usedDynamic Random Access Memory (DRAM), Static RAM (SRAM) and Read OnlyMemory (ROM) devices), a large number of bits are accessed in paralleleach time the system carries out a memory access cycle. However, only asmall percentage of accessed bits which are available internally eachtime the memory device is cycled ever make it across the device boundaryto the external world.

Referring to FIG. 1, all modern DRAM, SRAM and ROM designs have internalarchitectures with row (word) lines 5 and column (bit) lines 6 to allowthe memory cells to tile a two dimensional area 1. One bit of data isstored at the intersection of each word and bit line. When a particularword line is enabled, all of the corresponding data bits are transferredonto the bit lines. Some prior art DRAMs take advantage of thisorganization to reduce the number of pins needed to transmit theaddress. The address of a given memory cell is split into two addresses,row and column, each of which can be multiplexed over a bus only half aswide as the memory cell address of the prior art would have required.

COMPARISON WITH PRIOR ART

Prior art memory systems have attempted to solve the problem of highspeed access to memory with limited success. U.S. Pat. No. 3,821,715(Hoff et. al.), was issued to Intel Corporation for the earliest 4-bitmicroprocessor. That patent describes a bus connecting a single centralprocessing unit (CPU) with multiple RAMs and ROMs. That bus multiplexesaddresses and data over a 4-bit wide bus and uses point-to-point controlsignals to select particular RAMs or ROMs. The access time is fixed andonly a single processing element is permitted. There is no block-modetype of operation, and most important, not all of the interface signalsbetween the devices are bused (the ROM and RAM control lines and the RAMselect lines are point-to-point).

In U.S. Pat. No. 4,315,308 (Jackson), a bus connecting a single CPU to abus interface unit is described. The invention uses multiplexed address,data, and control information over a single 16-bit wide bus. Block-modeoperations are defined, with the length of the block sent as part of thecontrol sequence. In addition, variable access-time operations using a"stretch" cycle signal are provided. There are no multiple processingelements and no capability for multiple outstanding requests, and again,not all of the interface signals are bused.

In U.S. Pat. No. 4,449,207 (Kung, et. al.), a DRAM is described whichmultiplexes address and data on an internal bus. The external interfaceto this DRAM is conventional, with separate control, address and dataconnections.

In U.S. Pat. Nos. 4,764,846 and 4,706,166 (Go), a 3-D packagearrangement of stacked die with connections along a single edge isdescribed. Such packages are difficult to use because of thepoint-to-point wiring required to interconnect conventional memorydevices with processing elements. Both patents describe complex schemesfor solving these problems. No attempt is made to solve the problem bychanging the interface.

In U.S. Pat. No. 3,969,706 (Proebsting, et. al.), the currentstate-of-the-art DRAM interface is described. The address is two-waymultiplexed, and there are separate pins for data and control (RAS, CAS,WE, CS). The number of pins grows with the size of the DRAM, and many ofthe connections must be made point-to-point in a memory system usingsuch DRAMS.

There are many backplane buses described in the prior art, but not inthe combination described or having the features of this invention. Manybackplane buses multiplex addresses and data on a single bus (e.g., theNU bus). ELXSI and others have implemented split-transaction buses (U.S.Pat. Nos. 4,595,923 and 4,481,625 (Roberts)). ELXSI has also implementeda relatively low-voltage-swing current-mode ECL driver (approximately 1V swing). Address-space registers are implemented on most backplanebuses, as is some form of block mode operation.

Nearly all modern backplane buses implement some type of arbitrationscheme, but the arbitration scheme used in this invention differs fromeach of these. U.S. Pat. Nos. 4,837,682 (Culler), 4,818,985 (Ikeda),4,779,089 (Theus) and 4,745,548 (Blahut) describe prior art schemes. Allinvolve either log N extra signals, (Theus, Blahut), where N is thenumber of potential bus requestors, or additional delay to get controlof the bus (Ikeda, Culler). None of the buses described in patents orother literature use only bused connections. All contain somepoint-to-point connections on the backplane. None of the other aspectsof this invention such as power reduction by fetching each data blockfrom a single device or compact and low-cost 3-D packaging even apply tobackplane buses.

The clocking scheme used in this invention has not been used before andin fact would be difficult to implement in backplane buses due to thesignal degradation caused by connector stubs. U.S. Pat. No. 4,247,817(Heller) describes a clocking scheme using two clock lines, but relieson ramp-shaped clock signals in contrast to the normal rise-time signalsused in the present invention.

In U.S. Pat. No. 4,646,270 (Voss), a video RAM is described whichimplements a parallel-load, serial-out shift register on the output of aDRAM. This generally allows greatly improved bandwidth (and has beenextended to 2, 4 and greater width shift-out paths.) The rest of theinterfaces to the DRAM (RAS, CAS, multiplexed address, etc.) remain thesame as for conventional DRAMS.

One object of the present invention is to use a new bus interface builtinto semiconductor devices to support high-speed access to large blocksof data from a single memory device by an external user of the data,such as a microprocessor, in an efficient and cost-effective manner.

Another object of this invention is to provide a clocking scheme topermit high speed clock signals to be sent along the bus with minimalclock skew between devices.

Another object of this invention is to allow mapping out defectivememory devices or portions of memory devices.

Another object of this invention is to provide a method fordistinguishing otherwise identical devices by assigning a uniqueidentifier to each device.

Yet another object of this invention is to provide a method fortransferring address, data and control information over a relativelynarrow bus and to provide a method of bus arbitration when multipledevices seek to use the bus simultaneously.

Another object of this invention is to provide a method of distributinga high-speed memory cache within the DRAM chips of a memory system whichis much more effective than previous cache methods.

Another object of this invention is to provide devices, especiallyDRAMS, suitable for use with the bus architecture of the invention.

SUMMARY OF INVENTION

The present invention includes a memory subsystem comprising at leasttwo semiconductor devices, including at least one memory device,connected in parallel to a bus, where the bus includes a plurality ofbus lines for carrying substantially all address, data and controlinformation needed by said memory devices, where the control informationincludes device-select information and the bus has substantially fewerbus lines than the number of bits in a single address, and the buscarries device-select information without the need for separatedevice-select lines connected directly to individual devices.

Referring to FIG. 2, a standard DRAM 13, 14, ROM (or SRAM) 12,microprocessor CPU 11, I/O device, disk controller or other specialpurpose device such as a high speed switch is modified to use a whollybus-based interface rather than the prior art combination ofpoint-to-point and bus-based wiring used with conventional versions ofthese devices. The new bus includes clock signals, power and multiplexedaddress, data and control signals. In a preferred implementation, 8 busdata lines and an AddressValid bus line carry address, data and controlinformation for memory addresses up to 40 bits wide. Persons skilled inthe art will recognize that 16 bus data lines or other numbers of busdata lines can be used to implement the teaching of this invention. Thenew bus is used to connect elements such as memory, peripheral, switchand processing units.

In the system of this invention, DRAMs and other devices receive addressand control information over the bus end transmit or receive requesteddata over the same bus. Each memory device contains only a single businterface with no other signal pins. Other devices that may be includedin the system can connect to the bus and other non-bus lines, such asinput/output lines. The bus supports large data block transfers andsplit transactions to allow a user to achieve high bus utilization. Thisability to rapidly read or write a large block of data to one singledevice at a time is an important advantage of this invention.

The DRAMs that connect to this bus differ from conventional DRAMs in anumber of ways. Registers are provided which may store controlinformation, device identification, device-type and other informationappropriate for the chip such as the address range for each independentportion of the device. New bus interface circuits must be added and theinternals of prior art DRAM devices need to be modified so they canprovide and accept data to and from the bus at the peak data rate of thebus. This requires changes to the column access circuitry in the DRAM,with only a minimal increase in die size. A circuit is provided togenerate a low skew internal device clock for devices on the bus, andother circuits provide for demultiplexing input and multiplexing outputsignals.

High bus bandwidth is achieved by running the bus at a very high clockrate (hundreds of MHz). This high clock rate is made possible by theconstrained environment of the bus. The-bus lines arecontrolled-impedance, doubly-terminated lines. For a data rate of 500MHz, the maximum bus propagation time is less than 1 ns (the physicalbus length is about 10 cm). In addition, because of the packaging used,the pitch of the pins can be very close to the pitch of the pads. Theloading on the bus resulting from the individual devices is very small.In a preferred implementation, this generally allows stub capacitancesof 1-2 pF and inductances of 0.5-2 nH. Each device 15, 16, 17, shown inFIG. 3, only has pins on one side and these pins connect directly to thebus 18. A transceiver device 19 can be included to interface multipleunits to a higher order bus through pins 20.

A primary result of the architecture of this invention is to increasethe bandwidth of DRAM access. The invention also reduces manufacturingand production costs, power consumption, and increases packing densityand system reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram which illustrates the basic 2-D organization ofmemory devices.

FIG. 2 is a schematic block diagram which illustrates the parallelconnection of all bus lines and the serial Reset line to each device inthe system.

FIG. 3 is a perspective view of a system of the invention whichillustrates the 3-D packaging of semiconductor devices on the primarybus.

FIG. 4 shows the format of a request packet.

FIG. 5 shows the format of a retry response from a slave.

FIG. 6 shows the bus cycles after a request packet collision occurs onthe bus and how arbitration is handled.

FIGS. 7a and 7b show the timing whereby signals from two devices canoverlap temporarily and drive the bus at the same time.

FIGS. 8a and 8b show the connection and timing between bus clocks anddevices on the bus.

FIG. 9 is a perspective view showing how transceivers can be used toconnect a number of bus units to a transceiver bus.

FIG. 10 is a block and schematic diagram of input/output circuitry usedto connect devices to the bus.

FIG. 11 is a schematic diagram of a clocked sense-amplifier used as abus input receiver.

FIG. 12 is a block diagram showing how the internal device clock isgenerated from two bus clock signals using a set of adjustable delaylines.

FIG. 13 is a timing diagram showing the relationship of signals in theblock diagram of FIG. 12.

FIG. 14 is timing diagram of a preferred means of implementing the resetprocedure of this invention.

FIG. 15 is a diagram illustrating the general organization of a 4 MbitDRAM divided into 8 subarrays.

FIG. 16 is a block diagram representation of the device interfacecircuitry and internal registers within each device illustrated in FIG.2.

DETAILED DESCRIPTION

The present invention is designed to provide a high speed, multiplexedbus for communication between processing devices and memory devices andto provide devices adapted for use in the bus system. The invention canalso be used to connect processing devices and other devices, such asI/O interfaces or disk controllers, with or without memory devices onthe bus. The bus consists of a relatively small number of linesconnected in parallel to each device on the bus. The bus carriessubstantially all address, data and control information needed bydevices for communication with other devices on the bus. In many systemsusing the present invention, the bus carries almost every signal betweenevery device in the entire system. There is no need for separatedevice-select lines since device-select information for each device onthe bus is carried over the bus. There is no need for separate addressand data lines because address and data information can be sent over thesame lines. Using the organization described herein, very largeaddresses (40 bits in the preferred implementation) and large datablocks (1024 bytes) can be sent over a small number of bus lines (8 plusone control line in the preferred implementation).

Virtually all of the signals needed by a computer system can be sentover the bus. Persons skilled in the art recognize that certain devices,such as CPUs, may be connected to other signal lines and possibly toindependent buses, for example a bus to an independent cache memory, inaddition to the bus of this invention. Certain devices, for examplecross-point switches, could be connected to multiple, independent busesof this invention. In the preferred implementation, memory devices areprovided that have no connections other than the bus connectionsdescribed herein and CPUs are provided that use the bus of thisinvention as the principal, if not exclusive, connection to memory andto other devices on the bus.

All modern DRAM, SRAM and ROM designs have internal architectures withrow (word) and column (bit) lines to efficiently tile a 2-D area.Referring to FIG. 1, one bit of data is stored at the intersection ofeach word line 5 and bit line 6. When a particular word line is enabled,all of the corresponding data bits are transferred onto the bit lines.This data, about 4000 bits at a time in a 4 MBit DRAM, is then loadedinto column sense amplifiers 3 and held for use by the I/O circuits.

In the invention presented here, the data from the sense amplifiers isenabled 32 bits at a time onto an internal device bus running atapproximately 125 MHz. This internal device bus moves the data to theperiphery of the devices where the data is multiplexed into an 8-bitwide external bus interface, running at approximately 500 MHz.

The bus architecture of this invention connects master or bus controllerdevices, such as CPUs, Direct Memory Access devices (DMAs) or FloatingPoint Units (FPUs), and slave devices, such as DRAM, SRAM or ROM memorydevices. A slave device responds to control signals; a master sendscontrol signals. Persons skilled in the art realize that some devicesmay behave as both master and slave at various times, depending on themode of operation and the state of the system. For example, a memorydevice will typically have only slave functions, while a DMA controller,disk controller or CPU may include both slave and master functions. Manyother semiconductor devices, including I/O devices, disk controllers, orother special purpose devices such as high speed switches can bemodified for use with the bus of this invention.

With reference to FIG. 16, each semiconductor device contains a set ofinternal registers 170, preferably including a device identification(device ID) register 171, a device-type descriptor register 174, controlregisters 175 and other registers containing other information relevantto that type of device. In a preferred implementation, semiconductordevices connected to the bus contain registers 172 which specify thememory addresses contained within that device and access-time registers173 which store a set of one or more delay times at which the device canor should be available to send or receive data.

Most of these registers can be modified and preferably are set as partof an initialization sequence that occurs when the system is powered upor reset. During the initialization sequence each device on the bus isassigned a unique device ID number, which is stored in the device IDregister 171. A bus master can then use these device ID numbers toaccess and set appropriate registers in other devices, includingaccess-time registers 173, control registers 175, and memory registers172, to configure the system. Each slave may have one or severalaccess-time registers 173 (four in a preferred embodiment). In apreferred embodiment, one access-time register in each slave ispermanently or semi-permanently programmed with a fixed value tofacilitate certain control functions. A preferred implementation of aninitialization sequence is described below in more detail.

All information sent between master devices and slave devices is sentover the external bus, which, for example, may be 8 bits wide. This isaccomplished by defining a protocol whereby a master device, such as amicroprocessor, seizes exclusive control of the external bus (i.e.,becomes the bus master) and initiates a bus transaction by sending arequest packet (a sequence of bytes comprising address and controlinformation) to one or more slave devices on the bus. An address canconsist of 16 to 40 or more bits according to the teachings of thisinvention. Each slave on the bus must decode the request packet to seeif that slave needs to respond to the packet. The slave that the packetis directed to must then begin any internal processes needed to carryout the requested bus transaction at the requested time. The requestingmaster may also need to transact certain internal processes before thebus transaction begins. After a specified access time the slave(s)respond by returning one or more bytes (8 bits) of data or by storinginformation made available from the bus. More than one access time canbe provided to allow different types of responses to occur at differenttimes.

A request packet and the corresponding bus access are separated by aselected number of bus cycles, allowing the bus to be used in theintervening bus cycles by the same or other masters for additionalrequests or brief bus accesses. Thus multiple, independent accesses arepermitted, allowing maximum utilization of the bus for transfer of shortblocks of data. Transfers of long blocks of data use the bus efficientlyeven without overlap because the overhead due to bus address, controland access times is small compared to the total time to request andtransfer the block.

Device Address Mapping

Another unique aspect of this invention is that each memory device is acomplete, independent memory subsystem with all the functionality of aprior art memory board in a conventional backplane-bus computer system.Individual memory devices may contain a single memory section or may besubdivided into more than one discrete memory section. Memory devicespreferably include memory address registers for each discrete memorysection. A failed memory device (or even a subsection of a device) canbe "mapped out" with only the loss of a small fraction of the memory,maintaining essentially full system capability. Mapping out bad devicescan be accomplished in two ways, both compatible with this invention.

The preferred method uses address registers in each memory device (orindependent discrete portion thereof) to store information which definesthe range of bus addresses to which this memory device will respond.This is similar to prior art schemes used in memory boards inconventional backplane bus systems. The address registers can include asingle pointer, usually pointing to a block of known size, a pointer anda fixed or variable block size value or two pointers, one pointing tothe beginning and one to the end (or to the "top" and "bottom") or eachmemory block. By appropriate settings of the address registers, a seriesof functional memory devices or discrete memory sections can be made torespond to a contiguous range of addresses, giving the system access toa contiguous block of good memory, limited primarily by the number ofgood devices connected to the bus. A block of memory in a first memorydevice or memory section can be assigned a certain range of addresses,then a block of memory in a next memory device or memory section can beassigned addresses starting with an address one higher (or lower,depending on the memory structure) than the last address of the previousblock.

Preferred devices for use in this invention include device-type registerinformation specifying the type of chip, including how much memory isavailable in what configuration on that device. A master can perform anappropriate memory test, such as reading and writing each memory cell inone or more selected orders, to test proper functioning of eachaccessible discrete portion of memory (based in part on information likedevice ID number and device-type) and write address values (up to 40bits in the preferred embodiment, 10¹² bytes), preferably contiguous,into device address-space registers. Non-functional or impaired memorysections can be assigned a special address value which the system caninterpret to avoid using that memory.

The second approach puts the burden of avoiding the bad devices on thesystem master or masters. CPUs and DMA controllers typically have somesort of translation look-aside buffers (TLBs) which map virtual tophysical (bus) addresses. With relatively simple software, the TLBs canbe programmed to use only working memory (data structures describingfunctional memories are easily generated). For masters which don'tcontain TLBs (for example, a video display generator), a small, simpleRAM can be used to map a contiguous range of addresses onto theaddresses of the functional memory devices.

Either scheme works and permits a system to have a significantpercentage of non-functional devices and still continue to operate withthe memory which remains. This means that systems built with thisinvention will have much improved reliability over existing systems,including the ability to build systems with almost no field failures.

Bus

The preferred bus architecture of this invention comprises 11 signals:BusData 0:7!; AddrValid; Clk1 and Clk2; plus an input reference leveland power and ground lines connected in parallel to each device. Signalsare driven onto the bus during conventional bus cycles. The notation"Signal i:j!" refers to a specific range of signals or lines, forexample, BusData 0:7! means BusData0, BusData1, . . . , BusData7. Thebus lines for BusData 0:7! signals form a byte-wide, multiplexeddata/address/control bus. AddrValid is used to indicate when the bus isholding a valid address request, and instructs a slave to decode the busdata as an address and, if the address is included on that slave, tohandle the pending request. The two clocks together provide asynchronized, high speed clock for all the devices on the bus. Inaddition to the bused signals, there is one other line (ResetIn,ResetOut) connecting each device in series for use during initializationto assign every device in the system a unique device ID number(described below in detail).

To facilitate the extremely high data rate of this external bus relativeto the gate delays of the internal logic, the bus cycles are groupedinto pairs of even/odd cycles. Note that all devices connected to a busshould preferably use the same even/odd labeling of bus cycles andpreferably should begin operations on even cycles. This is enforced bythe clocking scheme.

Protocol and Bus Operation

The bus uses a relatively simple, synchronous, split-transaction,block-oriented protocol for bus transactions. One of the goals of thesystem is to keep the intelligence concentrated in the masters, thuskeeping the slaves as simple as possible (since there are typically manymore slaves than masters). To reduce the complexity of the slaves, aslave should preferably respond to a request in a specified time,sufficient to allow the slave to begin or possibly complete adevice-internal phase including any internal actions that must precedethe subsequent bus access phase. The time for this bus access phase isknown to all devices on the bus--each master being responsible formaking sure that the bus will be free when the bus access begins. Thusthe slaves never worry about arbitrating for the bus. This approacheliminates arbitration in single master systems, and also makes theslave-bus interface simpler.

In a preferred implementation of the invention, to initiate a bustransfer over the bus, a master sends out a request packet, a contiguousseries of bytes containing address and control information. It ispreferable to use a request packet containing an even number of bytesand also preferable to start each packet on an even bus cycle.

The device-select function is handled using the bus data lines.AddrValid is driven, which instructs all slaves to decode the requestpacket address, determine whether they contain the requested address,and if they do, provide the data back to the master (in the case of aread request) or accept data from the master (in the case of a writerequest) in a data block transfer. A master can also select a specificdevice by transmitting a device ID number in a request packet. In apreferred implementation, a special device ID number is chosen toindicate that the packet should be interpreted by all devices on thebus. This allows a master to broadcast a message, for example to set aselected control register of all devices with the same value.

The data block transfer occurs later at a time specified in the requestpacket control information, preferably beginning on an even cycle. Adevice begins a data block transfer almost immediately with adevice-internal phase as the device initiates certain functions, such assetting up memory addressing, before the bus access phase begins. Thetime after which a data block is driven onto the bus lines is selectedfrom values stored in slave access-time registers 173. The timing ofdata for reads and writes is preferably the same; the only difference iswhich device drives the bus. For reads, the slave drives the bus and themaster latches the values from the bus. For writes the master drives thebus and the selected slave latches the values from the bus.

In a preferred implementation of this invention shown in FIG. 4, arequest packet 22 contains 6 bytes of data--4.5 address bytes and 1.5control bytes. Each request packet uses all nine bits of the multiplexeddata/address lines (AddrValid 23+BusData 0:7! 24) for all six bytes ofthe request packet. Setting 23 AddrValid=1 in an otherwise unused evencycle indicates the start of an request packet (control information). Ina valid request packet, AddrValid 27 must be 0 in the last byte.Asserting this signal in the last byte invalidates the request packet.This is used for the collision detection and arbitration logic(described below). Bytes 25-26 contain the first 35 address bits,Address 0:35!. The last byte contains AddrValid 27 (the invalidationswitch) and 28, the remaining address bits, Address 36:39!, andBlockSize 0:3! (control information).

The first byte contains two 4 bit fields containing control information,AccessType 0:3!, an op code (operation code) which, for example,specifies the type of access, and Master 0:3!, a position reserved forthe master sending the packet to include its master ID number. Onlymaster numbers 1 through 15 are allowed--master number 0 is reserved forspecial system commands. Any packet with Master 0:3!=0 is an invalid orspecial packet and is treated accordingly.

The AccessType field specifies whether the requested operation is a reador write and the type of access, for example, whether it is to thecontrol registers or other parts of the device, such as memory. In apreferred implementation, AccessType 0! is a Read/Write switch: if it isa 1, then the operation calls for a read from the slave (the slave toread the requested memory block and drive the memory contents onto thebus) if it is a 0, the operation calls for a write into the slave (theslave to read data from the bus and write it to memory). AccessType 1:3!provides up to 8 different access types for a slave. AccessType 1:2!preferably indicates the timing of the response, which is stored in anaccess-time register, AccessRegN. The choice of access-time register canbe selected directly by having a certain op code select that register,or indirectly by having a slave respond to selected op codes withpre-selected access times (see table below). The remaining bit,AccessType 3! may be used to send additional information about therequest to the slaves.

One special type of access is control register access, which involvesaddressing a selected register in a selected slave. In the preferredimplementation of this invention, AccessType 1:3! equal to zeroindicates a control register request and the address field of the packetindicates the desired control register. For example, the mostsignificant two bytes can be the device ID number (specifying whichslave is being addressed) and the least significant three bytes canspecify a register address and may also represent or include data to beloaded into that control register. Control register accesses are used toinitialize the access-time registers, so it is preferable to use a fixedresponse time which can be preprogrammed or even hard wired, for examplethe value in AccessReg0, preferably 8 cycles. Control register accesscan also be used to initialize or modify other registers, includingaddress registers.

The method of this invention provides for access mode controlspecifically for the DRAMs. One such access mode determines whether theaccess is page mode or normal RAS access. In normal mode (inconventional DRAMS and in this invention), the DRAM column sense amps orlatches have been precharged to a value intermediate between logical 0and 1. This precharging allows access to a row in the RAM to begin assoon as the access request for either inputs (writes) or outputs (reads)is received and allows the column sense amps to sense data quickly. Inpage mode (both conventional and in this invention), the DRAM holds thedata in the column sense amps or latches from the previous read or writeoperation. If a subsequent request to access data is directed to thesame row, the DRAM does not need to wait for the data to be sensed (ithas been sensed already) and access time for this data is much shorterthan the normal access time. Page mode generally allows much fasteraccess to data but to a smaller block of data (equal to the number ofsense amps). However, if the requested data is not in the selected row,the access time is longer than the normal access time, since the requestmust wait for the RAM to precharge before the normal mode access canstart. Two access-time registers in each DRAM preferably contain theaccess times to be used for normal and for page-mode accesses,respectively.

The access mode also determines whether the DRAM should precharge thesense amplifiers or should save the contents of the sense amps for asubsequent page mode access. Typical settings are "precharge afternormal access" and "save after page mode access" but "precharge afterpage mode access" or "save after normal access" are allowed, selectablemodes of operation. The DRAM can also be set to precharge the sense ampsif they are not accessed for a selected period of time.

In page mode, the data stored in the DRAM sense amplifiers may beaccessed within much less time than it takes to read out data in normalmode (˜10-20 nS vs. 40-100 nS). This data may be kept available for longperiods. However, if these sense amps (and hence bit lines) are notprecharged after an access, a subsequent access to a different memoryword (row) will suffer a precharge time penalty of about 40-100 nSbecause the sense amps must precharge before latching in a new value.

The contents of the sense amps thus may be held and used as a cache,allowing faster, repetitive access to small blocks of data. DRAM-basedpage-mode caches have been attempted in the prior art using conventionalDRAM organizations but they are not very effective because several chipsare required per computer word. Such a conventional page-mode cachecontains many bits (for example, 32 chips×4 Kbits) but has very fewindependent storage entries. In other words, at any given point in timethe sense amps hold only a few different blocks or memory "locales" (asingle block of 4K words, in the example above). Simulations have shownthat upwards of 100 blocks are required to achieve high hit rates (>90%of requests find the requested data already in cache memory) regardlessof the size of each block. See, for example, Anant Agarwal, et. al., "AnAnalytic Cache Model," ACM Transactions on Computer Systems, Vol. 7(2),pp. 184-215 (May 1989).

The organization of memory in the present invention allows each DRAM tohold one or more (4 for 4 MBit DRAMS) separately-addressed andindependent blocks of data. A personal computer or workstation with 100such DRAMs (i.e. 400 blocks or locales) can achieve extremely high, veryrepeatable hit rates (98-99% on average) as compared to the lower(50-80%), widely varying hit rates using DRAMS organized in theconventional fashion. Further, because of the time penalty associatedwith the deferred precharge on a "miss" of the page-mode cache, theconventional DRAM-based page-mode cache generally has been found to workless well than no cache at all.

For DRAM slave access, the access types are preferably used in thefollowing way:

    ______________________________________    AccessType 1:3!               Use            AccessTime    ______________________________________    0          Control Register                              Fixed, 8 AccessReg0!               Access    1          Unused         Fixed, 8 AccessReg0!    2-3        Unused         AccessReg1    4-5        Page Mode DRAM AccessReg2               access    6-7        Normal DRAM access                              AccessReg3    ______________________________________

Persons skilled in the art will recognize that a series of availablebits could be designated as switches for controlling these access modes.For example:

AccessType 2!=page mode/normal switch

AccessType 3!=precharge/save-data switch

BlockSize 0:3! specifies the size of the data block transfer. IfBlockSize 0! is 0, the remaining bits are the binary representation ofthe block size (0-7). If BlockSize 0! is 1, then the remaining bits givethe block size as a binary power of 2, from 8 to 1024. A zero-lengthblock can be interpreted-as a special command, for example, to refresh aDRAM without returning any data, or to change the DRAM from page mode tonormal access mode or vice-versa.

    ______________________________________    BlockSize 0:2!   Number of Bytes in Block    ______________________________________    0-7              0-7 respectively    8                8    9                16    10               32    11               64    12               128    13               256    14               512    15               1024    ______________________________________

Persons skilled in the art will recognize that other block size encodingschemes or values can be used.

In most cases, a slave will respond at the selected access time byreading or writing data from or to the bus over bus lines BusData 0:7!and AddrValid will be at logical 0. In a preferred embodiment,substantially each memory access will involve only a single memorydevice, that is, a single block will be read from or written to a singlememory device.

Retry Format

In some cases, a slave may not be able to respond correctly to arequest, e.g., for a read or write. In such a situation, the slaveshould return an error message, sometimes called a N(o)ACK(nowledge) orretry message. The retry message can include information about thecondition requiring a retry, but this increases system requirements forcircuitry in both slave and masters. A simple message indicating onlythat an error has occurred allows for a less complex slave, and themaster can take whatever action is needed to understand and correct thecause of the error.

For example, under certain conditions a slave might not be able tosupply the requested data. During a page-mode access, the DRAM selectedmust be in page mode and the requested address must match the address ofthe data held in the sense amps or latches. Each DRAM can check for thismatch during a page-mode access. If no match is found, the DRAM beginsprecharging and returns a retry message to the master during the firstcycle of the data block (the rest of the returned block is ignored). Themaster then must wait for the precharge time (which is set toaccommodate the type of slave in question, stored in a special register,PreChargeReg), and then resend the request as a normal DRAM access(AccessType=6 or 7).

In the preferred form of the present invention, a slave signals a retryby driving AddrValid true at the time the slave was supposed to beginreading or writing data. A master which expected to write to that slavemust monitor AddrValid during the write and take corrective action if itdetects a retry message. FIG. 5 illustrates the format of a retrymessage 28 which is useful for read requests, consisting of 23AddrValid=1 with Master 0:3!=0 in the first (even) cycle. Note thatAddrValid is normally 0 for data block transfers and that there is nomaster 0 (only 1 through 15 are allowed). All DRAMs and masters caneasily recognize such a packet as an invalid request packet, andtherefore a retry message. In this type of bus transaction all of thefields except for Master 0:3! and Add-Valid 23 may be used asinformation fields, although in the implementation described, thecontents are undefined. Persons skilled in the art recognize thatanother method of signifying a retry message is to add a DataInvalidline and signal to the bus. This signal could be asserted in the case ofa NACK.

Bus Arbitration

In the case of a single master, there are by definition no arbitrationproblems. The master-sends request packets and keeps track of periodswhen the bus will be busy in response to that packet. The master canschedule multiple requests so that the corresponding data blocktransfers do not overlap.

The bus architecture of this invention is also useful in configurationswith multiple masters. When two or more masters are on the same bus,each master must keep track of all the pending transactions, so eachmaster knows when it can send a request packet and access thecorresponding data block transfer. Situations will arise, however, wheretwo or more masters send a request packet at about the same time and themultiple requests must be detected, then sorted out by some sort of busarbitration.

There are many ways for each master to keep track of when the bus is andwill be busy. A simple method is for each master to maintain a bus-busydata structure, for example by maintaining two pointers, one to indicatethe earliest point in the future when the bus will be busy and the otherto indicate the earliest point in the future when the bus will be free,that is, the end of the latest pending data block transfer. Using thisinformation, each master can determine whether and when there is enoughtime to send a request packet (as described above under Protocol) beforethe bus becomes busy with another data block transfer and whether thecorresponding data block transfer will interfere with pending bustransactions. Thus each master must read every request packet and updateits bus-busy data structure to maintain information about when the busis and will be free.

With two or more masters on the bus, masters will occasionally transmitindependent request packets during the same bus cycle. Those multiplerequests will collide as each such master drives the bus simultaneouslywith different information, resulting in scrambled request informationand neither desired data block transfer. In a preferred form of theinvention, each device on the bus seeking to write a logical 1 on aBusData or AddrValid line drives that line with a current sufficient tosustain a voltage greater than or equal to the high-logic value for thesystem. Devices do not drive lines that should have a logical 0; thoselines are simply held at a voltage corresponding to a low-logic value.Each master tests the voltage on at least some, preferably all, bus dataand the AddrValid lines so the master can detect a logical `1` where theexpected level is `0` on a line that it does not drive during a givenbus cycle but another master does drive.

Another way to detect collisions is to select one or more bus lines forcollision signalling. Each master sending a request drives that line orlines and monitors the selected lines for more than the normal drivecurrent (or a logical value of ">1"), indicating requests by more thanone master. Persons skilled in the art will recognize that this can beimplemented with a protocol involving BusData and AddrValid lines orcould be implemented using an additional bus line.

In the preferred form of this invention, each master detects collisionsby monitoring lines which it does not drive to see if another master isdriving those lines. Referring to FIG. 4, the first byte of the requestpacket includes the number of each master attempting to use the bus(Master 0:3!). If two masters send packet requests starting at the samepoint in time, the master numbers will be logical "or"ed together by atleast those masters, and thus one or both of the masters, by monitoringthe data on the bus and comparing what it sent, can detect a collision.For instance if requests by masters number 2 (0010) and 5 (0101)collide, the bus will be driven with the value Master 0:3!=7(0010+0101=0111). Master number 5 will detect that the signal Master2!=1 and master 2 will detect that Master 1! and Master 3!=1, tellingboth masters that a collision has occurred. Another example is masters 2and 11, for which the bus will be driven with the value Master 0:3!=11(0010+1011=1011), and although master 11 can't readily detect thiscollision, master 2 can. When any collision is detected, each masterdetecting a collision drives the value of AddrValid 27 in byte 5 of therequest packet 22 to 1, which is detected by all masters, includingmaster 11 in the second example above, and forces a bus arbitrationcycle, described below.

Another collision condition may arise where master A sends a requestpacket in cycle 0 and master B tries to send a request packet startingin cycle 2 of the first request packet, thereby overlapping the firstrequest packet. This will occur from time to time because the busoperates at high speeds, thus the logic in a second-initiating mastermay not be fast enough to detect a request initiated by a first masterin cycle 0 and to react fast enough by delaying its own request. MasterB eventually notices that it wasn't supposed to try to send a requestpacket (and consequently almost surely destroyed the address that masterA was trying to send), and, as in the example above of a simultaneouscollision, drives a 1 on AddrValid during byte 5 of the first requestpacket 27 forcing an arbitration. The logic in the preferredimplementation is fast enough that a master should detect a requestpacket by another master by cycle 3 of the first request packet, so nomaster is likely to attempt to send a potentially colliding requestpacket later than cycle 2.

Slave devices do not need to detect a collision directly, but they mustwait to do anything irrecoverable until the last byte (byte 5) is readto ensure that the packet is valid. A request packet with Master 0:3!equal to 0 (a retry signal) is ignored and does not cause a collision.The subsequent bytes of such a packet are ignored.

To begin arbitration after a collision, the masters wait a preselectednumber of cycles after the aborted request packet (4 cycles in apreferred implementation), then use the next free cycle to arbitrate forthe bus (the next available even cycle in the preferred implementation).Each colliding master signals to all other colliding masters that itseeks to send a request packet, a priority is assigned to each of thecolliding masters, then each master is allowed to make its request inthe order of that priority.

FIG. 6 illustrates one preferred way of implementing this arbitration.Each colliding master signals its intent to send a request packet bydriving a single BusData line during a single bus cycle corresponding toits assigned master number (1-15 in the present example). Duringtwo-byte arbitration cycle 29, byte 0 is allocated to requests 1-7 frommasters 1-7, respectively, (bit 0 is not used) and byte 1 is allocatedto requests 8-15 from masters 8-15, respectively. At least one deviceand preferably each colliding master reads the values on the bus duringthe arbitration cycles to determine and store which masters desire touse the bus. Persons skilled in the art will recognize that a singlebyte can be allocated for arbitration requests if the system includesmore bus lines than masters. More than 15 masters can be accommodated byusing additional bus cycles.

A fixed priority scheme (preferably using the master numbers, selectinglowest numbers first) is then used to prioritize, then sequence therequests in a bus arbitration queue which is maintained by at least onedevice. These requests are queued by each master in the bus-busy datastructure and no further requests are allowed until the bus arbitrationqueue is cleared. Persons skilled in the art will recognize that otherpriority schemes can be used, including assigning priority according tothe physical location of each master.

System Configuration/Reset

In the bus-based system of this invention, a mechanism is provided togive each device on the bus a unique device identifier (device ID) afterpower-up or under other conditions as desired or needed by the system. Amaster can then use this device ID to access a specific device,particularly to set or modify registers 170 of the specified device,including the control and address registers. In the preferredembodiment, one master is assigned to carry out the entire systemconfiguration process. The master provides a series of unique device IDnumbers for each unique device connected to the bus system. In thepreferred embodiment, each device connected to the bus contains aspecial device-type register which specifies the type of device, forinstance CPU, 4 MBit memory, 64 MBit memory or disk controller. Theconfiguration master should check each device, determine the device typeand set appropriate control registers, including access-time registers173. The configuration master should check each memory device and setall appropriate memory address registers 172.

One means to set up unique device ID numbers is to have each device toselect a device ID in sequence and store-the value in an internal deviceID register 171. For example, a master can pass sequential device IDnumbers through shift registers in each of a series of devices, or passa token from device to device whereby the device with the token reads indevice ID information from another line or lines. In a preferredembodiment, device ID numbers are assigned to devices according to theirphysical relationship, for instance, their order along the bus.

In a preferred embodiment of this invention, the device ID setting isaccomplished using a pair of pins on each device, ResetIn and ResetOut.These pins handle normal logic signals and are used only during deviceID configuration. On each rising edge of the clock, each device copiesResetIn (an input) into a four-stage reset shift register. The output ofthe reset shift register is connected to ResetOut, which in turnconnects to ResetIn for the next sequentially connected device.Substantially all devices on the bus are thereby daisy-chained together.A first reset signal, for example, while ResetIn at a device is alogical 1, or when a selected bit of the reset shift register goes fromzero to non-zero, causes the device to hard reset, for example byclearing all internal registers and resetting all state machines. Asecond reset signal, for example, the falling edge of ResetIn combinedwith changeable values on the external bus, causes that device to latchthe contents of the external bus into the internal device ID register(Device 0:7!).

To reset all devices on a bus, a master sets the ResetIn line of thefirst device to a "1" for long enough to ensure that all devices on thebus have been reset (4 cycles times the number of devices--note that themaximum number of devices on the preferred bus configuration is 256 (8bits), so that 1024 cycles is always enough time to reset all devices.)Then ResetIn is dropped to "0" and the BusData lines are driven with thefirst followed by successive device ID numbers, changing after every 4clock pulses. Successive devices set those device ID numbers into thecorresponding device ID register as the falling edge of ResetInpropagates through the shift registers of the daisy-chained devices.FIG. 14 shows ResetIn at a first device going low while a master drivesa first device ID onto the bus data lines BusData 0:3!. The first devicethen latches in that first device ID. After four clock cycles, themaster changes BusData 0:3! to the next device ID number and ResetOut atthe first device goes low, which pulls ResetIn for the nextdaisy-chained device low, allowing the next device to latch in the nextdevice ID number from BusData 0:3!. In the preferred embodiment, onemaster is assigned device ID 0 and it is the responsibility of thatmaster to control the ResetIn line and to drive successive device IDnumbers onto the bus at the appropriate times. In the preferredembodiment, each device waits two clock cycles after ResetIn goes lowbefore latching in a device ID number from BusData 0:3!.

Persons skilled in the art recognize that longer device ID numbers couldbe distributed to devices by having each device read in multiple bytesfrom the bus and latch the values into the device ID register. Personsskilled in the art also recognize that there are alternative ways ofgetting device ID numbers to unique devices. For instance, a series ofsequential numbers could be clocked along the ResetIn line and at acertain time each device could be instructed to latch the current resetshift register value into the device ID register 171.

The configuration master should choose and set an access time in eachaccess-time register 173 in each slave to a period sufficiently long toallow the slave to perform an actual, desired memory access. Forexample, for a normal DRAM access, this time must be longer than the rowaddress strobe (RAS) access time. If this condition is not met, theslave may not deliver the correct data. The value stored in a slaveaccess-time register 173 is preferably one-half the number of bus cyclesfor which the slave device should wait before using the bus in responseto a request. Thus an access time value of `1` would indicate that theslave should not access the bus until at least two cycles after the lastbyte of the request packet has been received. The value of AccessReg0 ispreferably fixed at 8 (cycles) to facilitate access to controlregisters.

The bus architecture of this invention can include more than one masterdevice. The reset or initialization sequence should also include adetermination of whether there are multiple masters on the bus, and ifso to assign unique master ID numbers to each. Persons skilled in theart will recognize that there are many ways of doing this. For instance,the master could poll each device to determine what type of device itis, for example, by reading a special register then, for each masterdevice, write the next available master ID number into a specialregister.

ECC

Error detection and correction ("ECC") methods well known in the art canbe implemented in this system. ECC information typically is calculatedfor a block of data at the time that block of data is first written intomemory. The data block usually has an integral binary size, e.g. 256bits, and the ECC information uses significantly fewer bits. A potentialproblem arises in that each binary data block in prior art schemestypically is stored with the ECC bits appended, resulting in a blocksize that is not an integral binary power.

In a preferred embodiment of this invention, ECC information is storedseparately from the corresponding data, which can then be stored inblocks having integral binary size. ECC information and correspondingdata can be stored, for example, in separate DRAM devices. Data can beread without ECC using a single request packet, but to write or readerror-corrected data requires two request packets, one for the data anda second for the corresponding ECC information. ECC information may notalways be stored permanently and in some situations the ECC informationmay be available without sending a request packet or without a bus datablock transfer.

In a preferred embodiment, a standard data block size can be selectedfor use with ECC, and the ECC method will determine the required numberof bits of information in a corresponding ECC block. RAMs containing ECCinformation can be programmed to store an access time that is equal to:(1) the access time of the normal RAM (containing data) plus the time toaccess a standard data block (for corrected data) minus the time to senda request packet (6 bytes); or (2) the access time of a normal RAM minusthe time to access a standard ECC block minus the time to send a requestpacket. To read a data block and the corresponding ECC block, the mastersimply issues a request for the data immediately followed by a requestfor the ECC block. The ECC RAM will wait for the selected access timethen drive its data onto the bus right after (in case (1) above)) thedata RAM has finished driving out the data block. Persons skilled in theart will recognize that the access time described in case (2) above canbe used to drive ECC data before the data is driven onto the bus linesand will recognize that writing data can be done by analogy with themethod described for a read. Persons skilled in the art will alsorecognize the adjustments that must be made in the bus-busy structureand the request packet arbitration methods of this invention in order toaccommodate these paired ECC requests.

Since this system is quite flexible, the system designer can choose thesize of the data blocks and the number of ECC bits using the memorydevices of this invention. Note that the data stream on the bus can beinterpreted in various ways. For instance the sequence can be 2^(n) databytes followed by 2^(m) ECC bytes (or vice versa), or the sequence canbe 2^(k) iterations of 8 data bytes plus 1 ECC byte. Other information,such as information used by a directory-based cache coherence scheme,can also be managed this way. See, for example, Anant Agarwal, et al.,"Scaleable Directory Schemes for Cache Consistency," 15th InternationalSymposium on Computer Architecture, June 1988, pp. 280-289. Thoseskilled in the art will recognize alternative methods of implementingECC schemes that are within the teachings of this invention.

Low Power 3-D Packaging

Another major advantage of this invention is that it drastically reducesthe memory system power consumption. Nearly all the power consumed by aprior art DRAM is dissipated in performing row access. By using a singlerow access in a single RAM to supply all the bits for a block request(compared to a row-access in each of multiple RAMs in conventionalmemory systems) the power per bit can be made very small. Since thepower dissipated by memory devices using this invention is significantlyreduced, the devices potentially can be placed much closer together thanwith conventional designs.

The bus architecture of this invention makes possible an innovative 3-Dpackaging technology. By using a narrow, multiplexed (time-shared) bus,the pin count for an arbitrarily large memory device can be kept quitesmall--on the order of 20 pins. Moreover, this pin count can be keptconstant from one generation of DRAM density to the next. The low powerdissipation allows each package to be smaller, with narrower pin pitches(spacing between the IC pins). With current surface mount technologysupporting pin pitches as low as 20 mils, all off-device connections canbe implemented on a single edge of the memory device. Semiconductor dieuseful in this invention preferably have connections or pads along oneedge of the die which can then be wired or otherwise connected to thepackage pins with wires having similar lengths. This geometry alsoallows for very short leads, preferably with an effective lead length ofless than 4 mm. Furthermore, this invention uses only busedinterconnections, i.e., each pad on each device is connected by the busto the corresponding pad of each other device.

The use of a low pin count and an edge-connected bus permits a simple3-D package, whereby the devices are stacked and the bus is connectedalong a single edge of the stack. The fact that all of the signals arebused is important for the implementation of a simple 3-D structure.Without this, the complexity of the "backplane" would be too difficultto make cost effectively with current technology. The individual devicesin a stack of the present invention can be packed quite tightly becauseof the low power dissipated by the entire memory system, permitting thedevices to be stacked bumper-to-bumper or top to bottom. Conventionalplastic-injection molded small outline (SO) packages can be used with apitch of about 2.5 mm (100 mils), but the ultimate limit would be thedevice die thickness, which is about an order of magnitude smaller,0.2-0.5 mm using current wafer technology.

Bus Electrical Description

By using devices with very low power dissipation and close physicalpacking, the bus can be made quite short, which in turn allows for shortpropagation times and high data rates. The bus of a preferred embodimentof the present invention consists of a set of resistor-terminatedcontrolled impedance transmission lines which can operate up to a datarate of 500 MHz (2 ns cycles). The characteristics of the transmissionlines are strongly affected by the loading caused by the DRAMs (or otherslaves) mounted on the bus. These devices add lumped capacitance to thelines which both lowers the impedance of the lines and decreases thetransmission speed. In the loaded environment, the bus impedance islikely to be on the order of 25 ohms and the propagation velocity aboutc/4 (c=the speed of light) or 7.5 cm/ns. To operate at a 2 ns data rate,the transit time on the bus should preferably be kept under 1 ns, toleave 1 ns for the setup and hold time of the input receivers (describedbelow) plus clock skew. Thus the bus lines must be kept quite short,under about 8 cm for maximum performance. Lower performance systems mayhave much longer lines, e.g. a 4 ns bus may have 24 cm lines (3 nstransit time, 1 ns setup and hold time).

In the preferred embodiment, the bus uses current source drivers. Eachoutput rust be able to sink 50 mA, which provides an output swing ofabout 500 mV or more. In the preferred embodiment of this invention, thebus is active low. The unasserted state (the high value) is preferablyconsidered a logical zero, and the asserted value (low state) istherefore a logical 1. Those skilled in the art understand that themethod of this invention can also be implemented using the oppositelogical relation to voltage. The value of the unasserted state is set bythe voltage on the termination resistors, and should be high enough toallow the outputs to act as current sources, while being as low aspossible to reduce power dissipation. These constraints may yield atermination voltage about 2V above ground in the preferredimplementation. Current source drivers cause the output voltage to beproportional to the sum of the sources driving the bus.

Referring to FIGS. 7a and 7b, although there is no stable conditionwhere two devices drive the bus at the same time, conditions can arisebecause of propagation delay on the wires where one device, A 41, canstart driving its part of the bus 44 while the bus is still being drivenby another device, B 42 (already asserting a logical 1 on the bus). In asystem using current drivers, when B 42 is driving the bus (before time46), the value at points 44 and 45 is logical 1. If B 42 switches off attime 46 just when A 41 switches on, the additional drive by device A 41causes the voltage at the output 44 of A 41 to drop briefly below thenormal value. The voltage returns to its normal value at time 47 whenthe effect of device B 42 turning off is felt. The voltage at point 45goes to logical 0 when device B 42 turns off, then drops at time 47 whenthe effect of device A 41 turning on is felt. Since the logical 1 drivenby current from device A 41 is propagated irrespective of the previousvalue on the bus, the value on the bus is guaranteed to settle after onetime of flight (t_(f)) delay, that is, the time it takes a signal topropagate from one end of the bus to the other. If a voltage drive wasused (as in ECL wired-ORing), a logical 1 on the bus (from device B 42being previously driven) would prevent the transition put out by deviceA 41 being felt at the most remote part of the system, e.g., device 43,until the turnoff waveform from device B 42 reached device A 41 plus onetime of flight delay, giving a worst case settling time of twice thetime of flight delay.

Clocking

Clocking a high speed bus accurately without introducing error due topropagation delays can be implemented by having each device monitor twobus clock signals and then derive internally a device clock, the truesystem clock. The bus clock information can be sent on one or two linesto provide a mechanism for each bused device to generate an internaldevice clock with zero skew relative to all the other device clocks.Referring to FIG. 8a, in the preferred implementation, a bus clockgenerator 50 at one end of the bus propagates an early bus clock signalin one direction along the bus, for example on line 53 from right toleft, to the far end of the bus. The same clock signal then is passedthrough the direct connection shown to a second line 54, and returns asa late bus clock signal along the bus from the far end to the origin,propagating from left to right. A single bus clock line can be used ifit is left unterminated at the far end of the bus, allowing the earlybus clock signal to reflect back along the same line as a late bus clocksignal.

FIG. 8b illustrates how each device 51, 52 receives each of the two busclock signals at a different time (because of propagation delay alongthe wires), with constant midpoint in time between the two bus clocksalong the bus. At each device 51, 52, the rising edge 55 of Clock1 53 isfollowed by the rising edge 56 of Clock2 54. Similarly, the falling edge57 of Clock1 53 is followed by the falling edge 58 of Clock2 54. Thiswaveform relationship is observed at all other devices along the bus.Devices which are closer to the clock generator have a greaterseparation between Clock1 and Clock2 relative to devices farther fromthe generator because of the longer time required for each clock pulseto traverse the bus and return along line 54, but the midpoint in time59, 60 between corresponding rising or falling edges is fixed because,for any given device, the length of each clock line between the far endof the bus and that device is equal. Each device must sample the two busclocks and generate its own internal device clock at the midpoint of thetwo.

Clock distribution problems can be further reduced by using a bus clockand device clock rate equal to the bus cycle data rate divided by two,that is, the bus clock period is twice the bus cycle period. Thus a 500MHz bus preferably uses a 250 MHz clock rate. This reduction infrequency provides two benefits. First it makes all signals on the bushave the same worst case data rates--data on a 500 MHz bus can onlychange every 2 ns. Second, clocking at half the bus cycle data ratemakes the labeling of the odd and even bus cycles trivial, for example,by defining even cycles to be those when the internal device clock is 0and odd cycles when the internal device clock is 1.

Multiple Buses

The limitation on bus length described above restricts the total numberof devices that can be placed on a single bus. Using 2.5 mm spacingbetween devices, a single 8 cm bus will hold about 32 devices. Personsskilled in the art will recognize certain applications of the presentinvention wherein the overall data rate on the bus is adequate butmemory or processing requirements necessitate a much larger number ofdevices (many more than 32). Larger systems can easily be built usingthe teachings of this invention by using one or more memory subsystems,designated primary bus units, each of which consists of two or moredevices, typically 32 or close to the maximum allowed by bus designrequirements, connected to a transceiver device.

Referring to FIG. 9, each primary bus unit can be mounted on a singlecircuit board 66, sometimes called a memory stick. Each transceiverdevice 19 in turn connects to a transceiver bus 65, similar or identicalin electrical and other respects to the primary bus 18 described atlength above. In a preferred implementation, all masters are situated onthe transceiver bus so there are no transceiver delays between mastersand all memory devices are on primary bus units so that all memoryaccesses experience an equivalent transceiver delay, but persons skilledin the art will recognize how to implement systems which have masters onmore than one bus unit and memory devices on the transceiver bus as wellas on primary bus units. In general, each teaching of this inventionwhich refers to a memory device can be practiced using a transceiverdevice and one or more memory devices on an attached primary bus unit.Other devices, generically referred to as peripheral devices, includingdisk controllers, video controllers or I/O devices can also be attachedto either the transceiver bus or a primary bus unit, as desired. Personsskilled in the art will recognize how to use a single primary bus unitor multiple primary bus units as needed with a transceiver bus incertain system designs.

The transceivers are quite simple in function. They detect requestpackets on the transceiver bus and transmit them to their primary busunit. If the request packet calls for a write to a device on atransceiver's primary bus unit, that transceiver keeps track of theaccess time and block size and forwards all data from the transceiverbus to the primary bus unit during that time. The transceivers alsowatch their primary bus unit, forwarding any data that occurs there tothe transceiver bus. The high speed of the buses means that thetransceivers will need to be pipelined, and will require an additionalone or two cycle delay for data to pass through the transceiver ineither direction. Access times stored in masters on the transceiver busmust be increased to account for transceiver delay but access timesstored in slaves on a primary bus unit should not be modified.

Persons skilled in the art will recognize that a more sophisticatedtransceiver can control transmissions to and from primary bus units. Anadditional control line, TrncvrRW can be bused to all devices on thetransceiver bus, using that line in conjunction with the AddrValid lineto indicate to all devices on the transceiver bus that the informationon the data lines is: 1) a request packet, 2) valid data to a slave, 3)valid data from a slave, or 4) invalid data (or idle bus). Using thisextra control line obviates the need for the transceivers to keep trackof when data needs to be forwarded from its primary bus to thetransceiver bus--all transceivers send all data from their primary busto the transceiver bus whenever the control signal indicates condition2) above. In a preferred implementation of this invention, if AddrValidand TrncvrRW are both low, there is no bus activity and the transceiversshould remain in an idle state. A controller sending a request packetwill drive AddrValid high, indicating to all devices on the transceiverbus that a request packet is being sent which each transceiver shouldforward to its primary bus unit. Each controller seeking to write to aslave should drive both AddrValid and TrncvrRW high, indicating validdata for a slave is present on the data lines. Each transceiver devicewill then transmit all data from the transceiver bus lines to eachprimary bus unit. Any controller expecting to receive information from aslave should also drive the TrncvrRW line high, but not drive AddrValid,thereby indicating to each transceiver to transmit any data coming fromany slave on its primary local bus to the transceiver bus. A still moresophisticated transceiver would recognize signals addressed to or comingfrom its primary bus unit and transmit signals only at requested times.

An example of the physical mounting of the transceivers is shown in FIG.9. One important feature of this physical arrangement is to integratethe bus of each transceiver 19 with the original bus of DRAMs or otherdevices 15, 16, 17 on the primary bus unit 66. The transceivers 19 havepins on two sides, and are preferably mounted flat on the primary busunit with a first set of pins connected to primary bus 18. A second setof transceiver pins 20, preferably orthogonal to the first set of pins,are oriented to allow the transceiver 19 to be attached to thetransceiver bus 65 in much the same way as the DRAMs were attached tothe primary bus unit. The transceiver bus can be generally planar and ina different plane, preferably orthogonal to the plane of each primarybus unit. The transceiver bus can also be generally circular withprimary bus units mounted perpendicular and tangential to thetransceiver bus.

Using this two level scheme allows one to easily build a system thatcontains over 500 slaves (16 buses of 32 DRAMs each). Persons skilled inthe art can modify the device ID scheme described above to accommodatemore than 256 devices, for example by using a longer device ID or byusing additional registers to hold some of the device ID. This schemecan be extended in yet a third dimension to make a second-ordertransceiver bus, connecting multiple transceiver buses by aligningtransceiver bus units parallel to and on top of each other and busingcorresponding signal lines through a suitable transceiver. Using such asecond-order transceiver bus, one could connect many thousands of slavedevices into what is effectively a single bus.

Device Interface

The device interface 161 to the high-speed bus can be divided into threemain parts. The first part is the electrical interface 190. This partincludes the input receivers, bus drivers and clock generationcircuitry. The second part contains the address comparison circuitry andtiming registers. This part takes the input request packet anddetermines if the request is for this device, and if it is, starts theinternal access and delivers the data to the pins at the correct time.The final part, specifically for memory devices such as DRAMs, is theDRAM column access path. This part needs to provide bandwidth into andout of the DRAM sense amps greater than the bandwidth provided byconventional DRAMs. The implementation of the electrical interface andDRAM column access path are described in more detail in the followingsections. Persons skilled in the art recognize how to modify prior-artaddress comparison circuitry and prior-art register circuitry in orderto practice the present invention.

Electrical Interface--Input/Output Circuitry

A block diagram of the preferred input/output circuit foraddress/data/control lines is shown in FIG. 10. This circuitry isparticularly well-suited for use in DRAM devices but it can be used ormodified by one skilled in the art for use in other devices connected tothe bus of this invention. It consists of a set of input receivers 71,72 and output driver 76 connected to input/output line 69 and pad 75 andcircuitry to use the internal clock 73 and internal clock complement 74to drive the input interface. The clocked input receivers take advantageof the synchronous nature of the bus. To further reduce the performancerequirements for device input receivers, each device pin, and thus eachbus line, is connected to two clocked receivers, one to sample the evencycle inputs, the other to sample the odd cycle inputs. By thusde-multiplexing the input 69 at the pin, each clocked amplifier is givena full 2 ns cycle to amplify the bus low-voltage-swing signal into afull value CMOS logic signal. Persons skilled in the art will recognizethat additional clocked input receivers can be used within the teachingsof this invention. For example, four input receivers could be connectedto each device pin and clocked by a modified internal device clock totransfer sequential bits from the bus to internal device circuits,allowing still higher external bus speeds or still longer settling timesto amplify the bus low-voltage-swing signal into a full value CMOS logicsignal.

The output drivers are quite simple, and consist of a single NMOSpulldown transistor 76. This transistor is sized so that under worstcase conditions it can still sink the 50 mA required by the bus. For 0.8micron CMOS technology, the transistor will need to be about 200 micronslong. Overall bus performance can be improved by using feedbacktechniques to control output transistor current so that the currentthrough the device is roughly 50 mA under all operating conditions,although this is not absolutely necessary for proper bus operation. Anexample of one of many methods known to persons skilled in the art forusing feedback techniques to control current is described in HansSchumacher, et al., "CMOS Subnanosecond True-ECL Output Buffer," J.Solid State Circuits, Vol. 25 (1), pp. 150-154 (February 1990).Controlling this current improves performance and reduces powerdissipation. This output driver which can be operated at 500 MHz, can inturn be controlled by a suitable multiplexer with two or more(preferably four) inputs connected to other internal chip circuitry, allof which can be designed according to well known prior art.

The input receivers of every slave must be able to operate during everycycle to determine whether the signal on the bus is a valid requestpacket. This requirement leads to a number of constraints on the inputcircuitry. In addition to requiring small acquisition and resolutiondelays, the circuits must take little or no DC power, little AC powerand inject very little current back into the input or reference lines.The standard clocked DRAM sense amp shown in FIG. 11 satisfies all theserequirements except the need for low input currents. When this sense ampgoes from sense to sample, the capacitance of the internal nodes 83 and84 in FIG. 11 is discharged through the reference line 68 and input 69,respectively. This particular current is small, but the sum of suchcurrents from all the inputs into the reference lines summed over alldevices can be reasonably large.

The fact that the sign of the current depends upon on the previousreceived data makes matters worse. One way to solve this problem is todivide the sample period into two phases. During the first phase, theinputs are shorted to a buffered version of the reference level (whichmay have an offset). During the second phase, the inputs are connectedto the true inputs. This scheme does not remove the input currentcompletely, since the input must still charge nodes 83 and 84 from thereference value to the current input value, but it does reduce the totalcharge required by about a factor of 10 (requiring only a 0.25V changerather than a 2.5V change). Persons skilled in the art will recognizethat many other methods can be used to provide a clocked amplifier thatwill operate on very low input currents.

One important part of the input/output circuitry generates an internaldevice clock based on early and late bus clocks. Controlling clock skew(the difference in clock timing between devices) is important in asystem running with 2 ns cycles, thus the internal device clock isgenerated so the input sampler and the output driver operate as close intime as possible to midway between the two bus clocks.

A block diagram of the internal device clock generating circuit is shownin FIG. 12 and the corresponding timing diagram in FIG. 13. The basicidea behind this circuit is relatively simple. A DC amplifier 102 isused to convert the small-swing bus clock into a full-swing CMOS signal.This signal is then fed into a variable delay line 103. The output ofdelay line 103 feeds three additional delay lines: 104 having a fixeddelay; 105 having the same fixed delay plus a second variable delay; and106 having the same fixed delay plus one half of the second variabledelay. The outputs 107, 108 of the delay lines 104 and 105 drive clockedinput receivers 101 and 111 connected to early and late bus clock inputs100 and 110, respectively. These input receivers 101 and 111 have thesame design as the receivers described above and shown in FIG. 11.Variable delay lines 103 and 105 are adjusted via feedback lines 116,115 so that input receivers 101 and 111 sample the bus clocks just asthey transition. Delay lines 103 and 105 are adjusted so that thefalling edge 120 of output 107 precedes the falling edge 121 of theearly bus clock, Clock1 53, by an amount of time 128 equal to the delayin input sampler 101. Delay line 108 is adjusted in the same way so thatfalling edge 122 precedes the falling edge 123 of late bus clock, Clock254, by the delay 128 in input sampler 111.

Since the outputs 107 and 108 are synchronized with the two bus clocksand the output 73 of the last delay line 106 is midway between outputs107 and 108, that is, output 73 follows output 107 by the same amount oftime 129 that output 73 precedes output 108, output 73 provides aninternal device clock midway between the bus clocks. The falling edge124 of internal device clock 73 precedes the time of actual inputsampling 125 by one sampler delay. Note that this circuit organizationautomatically balances the delay in substantially all device inputreceivers 71 and 72 (FIG. 10), since outputs 107 and 108 are adjusted sothe bus clocks are sampled by input receivers 101 and 111 just as thebus clocks transition.

In the preferred embodiment, two sets of these delay lines are used, oneto generate the true value of the internal device clock 73, and theother to generate the complement 74 without adding any inverter delay.The dual circuit allows generation of truly complementary clocks, withextremely small skew. The complement internal device clock is used toclock the `even` input receivers to sample at time 127, while the trueinternal device clock is used to clock the `odd` input receivers tosample at time 125. The true and complement internal device clocks arealso used to select which data is driven to the output drivers. The gatedelay between the internal device clock and output circuits driving thebus is slightly greater than the corresponding delay for the inputcircuits, which means that the new data always will be driven on the busslightly after the old data has been sampled.

DRAM Column Access Modification

A block diagram of a conventional 4 MBit DRAM 130 is shown in FIG. 15.The DRAM memory array is divided into a number of subarrays 150-157, forexample, 8. Each subarray is divided into arrays 148, 149 of memorycells. Row address selection is performed by decoders 146. A columndecoder 147A, 147B, including column sense amps on either side of thedecoder, runs through the core of each subarray. These column sense ampscan be set to precharge or latch the most-recently stored value, asdescribed in detail above. Internal I/O lines connect each set ofsense-amps, as gated by corresponding column decoders, to input andoutput circuitry connected ultimately to the device pins. These internalI/O lines are used to drive the data from the selected bit lines to thedata pins (some of pins 131-145), or to take the data from the pins andwrite the selected bit lines. Such a column access path organized byprior art constraints does not have sufficient bandwidth to interfacewith a high speed bus. The method of this invention does not requirechanging the overall method used for column access, but does changeimplementation details. Many of these details have been implementedselectively in certain fast memory devices, but never in conjunctionwith the bus architecture of this invention.

Running the internal I/O lines in the conventional way at high bus cyclerates is not possible. In the preferred method, several (preferably 4)bytes are read or written during each cycle and the column access pathis modified to run at a lower rate (the inverse of the number of bytesaccessed per cycle, preferably 1/4 of the bus cycle rate). Threedifferent techniques are used to provide the additional internal I/Olines required and to supply data to memory cells at this rate. First,the number of I/O bit lines in each subarray running through the columndecoder 147A,B is increased, for example, to 16, eight for each of thetwo columns of column sense amps and the column decoder selects one setof columns from the "top" half 148 of subarray 150 and one set ofcolumns from the "bottom" half 149 during each cycle, where the columndecoder selects one column sense amp per I/O bit line. Second, eachcolumn I/O line is divided into two halves, carrying data independentlyover separate internal I/O lines from the left half 147A and right half147B of each subarray (dividing each subarray into quadrant's) and thecolumn decoder selects sense amps from each right and left half of thesubarray, doubling the number of bits availably at each cycle. Thus eachcolumn decode selection turns on n column sense amps, where n equalsfour (top left and right, bottom left and right quadrants) times thenumber of I/O lines in the bus to each subarray quadrant (8 lineseach×4=32 lines in the preferred implementation). Finally, during eachRAS cycle, two different subarrays, e.g. 157 and 153, are accessed. Thisdoubles again the available number of I/O lines containing data. Takentogether, these changes increase the internal I/O bandwidth by at leasta factor of 8. Four internal buses are used to route these internal I/Olines. Increasing the number of I/O lines and then splitting them in themiddle greatly reduces the capacitance of each internal I/O line whichin turn reduces the column access time, increasing the column accessbandwidth even further.

The multiple, gated input receivers described above allow high speedinput from the device pins onto the internal I/O lines and ultimatelyinto memory. The multiplexed output driver described above is used tokeep up with the data flow available using these techniques. Controlmeans are provided to select whether information at the device pinsshould be treated as an address, and therefore to be decoded, or inputor output data to be driven onto or read from the internal I/O lines.

Each subarray can access 32 bits per cycle, 16 bits from the leftsubarray and 16 from the right subarray. With 8 I/O lines per senseamplifier column and accessing two subarrays at a time the DRAM canprovide 64 bits per cycle. This extra I/O bandwidth is not needed forreads (and is probably not used), but may be needed for writes.Availability of write bandwidth is a more difficult problem than readbandwidth because over-writing a value in a sense-amplifier may be aslow operation, depending on how the sense amplifier is connected to thebit line. The extra set of internal I/O lines provides some bandwidthmargin for write operations.

Persons skilled in the art will recognize that many variations of theteachings of this invention can be practiced that still fall within theclaims of this invention which follow.

What is claimed is:
 1. A method for assigning identification values tomemories, comprising the steps of:(1) having a master reset identifiersof a first memory and a second memory by sending a reset signal on aline that is coupled in a daisy-chained manner to the first and secondmemories and also coupled to the master; (2) having a master send firstand second identification values on the daisy-chained line to the firstand second memories; (3) having the first memory store at a firstpredetermined time the first identification value on the daisy-chainedline as an identifier for the first memory; (4) having the second memorystore at a second predetermined time the second identification value onthe daisy-chained line as an identifier for the second memory.
 2. Anapparatus for assigning identification values to memories, the apparatuscomprising:(1) a daisy-chained line coupled to a master, a first memoryand a second memory; (2) a data bus coupled to the master, the firstmemory, and the second memory; (3) reset circuitry for sending a resetsignal from the master to the first and second memories via thedaisy-chained line and for resetting identifiers of the first and secondmemories in response to the reset signal; (4) circuitry within themaster for generating first and second identification values and sendingthem to the first and second memories via the daisy-chained line; (5)circuitry within the first memory for storing at a first predeterminedtime the first identification value on the daisy-chained line as anidentifier for the first memory; and (6) circuitry within the secondmemory for storing at a second predetermined time the secondidentification value on the daisy-chained line as an identifier for thesecond memory.
 3. A memory module having a plurality of memory devices,the memory module comprises:a bus having a plurality of signal linesincluding a reset signal line which is electrically connected to theplurality of memory devices; a first memory device including:at leastone memory section having a plurality of memory cells which are accessedusing address signals; an internal register to store an identificationvalue to identify the first memory device on the bus; and interfacecircuitry, coupled to the bus, to receive a reset request via the resetsignal line wherein, in response to a received reset request, theinternal register in the first memory device stores the identificationvalue which identifies the first memory device on the bus; and a secondmemory device including:at least one memory section having a pluralityof memory cells which are accessed using address signals; an internalregister to store an identification value to identify the second memorydevice on the bus; and interface circuitry, coupled to the bus, toreceive a reset request via the reset signal line wherein, in responseto a received reset request, the internal register in the second memorydevice stores the identification value which identifies the secondmemory device on the bus.
 4. The memory module of claim 3 wherein:theinternal register of the first memory device stores a uniqueidentification value to identify the first memory device from thyplurality of memory devices, including the second memory device, on thebus; and the internal register of the second memory device stores aunique identification value to identify the second memory device fromthe plurality of memory devices, including the first memory device, onthe bus.
 5. The memory module of claim 3 wherein the internal registerof the first memory device stores an identification value to identifythe first memory device and a plurality of memory devices, including thesecond memory device, on the bus.
 6. The memory module of claim 3wherein the bus includes a clock signal line for receiving an externalclock signal and data lines for receiving the identification values. 7.The memory module of claim 6 wherein the first memory device includes areset register coupled to the reset signal line to receive the resetsignal.
 8. The memory module of claim 7 wherein the reset register ofthe first memory device is clocked synchronously with respect to theexternal clock signal such that the reset signal is clocked through thereset register.
 9. The memory module of claim 8 wherein theidentification register of the first memory device stores theidentification value while the reset signal is in the reset register ofthe first memory device.
 10. The memory module of claim 9 wherein theinternal register of the first memory device stores the identificationvalue of the first memory device register after a predetermined numberof clock cycles of the external clock signal and while the reset signalis in the reset register of the first memory device.
 11. The memorymodule of claim 3 wherein the first and second memory devices receivethe identification values on the reset signal line.
 12. The memorymodule of claim 11 wherein the internal register of the first memorydevice stores the identification value which identifies the first memorydevice on the bus at a first predetermined time and the internalregister of the second memory device stores the identification valuewhich identifies the second memory device on the bus at a secondpredetermined time.
 13. The memory module of claim 12 wherein the firstand the second predetermined times are the same.
 14. The memory moduleof claim 12 wherein the identification values are clocked along thereset signal line synchronously with respect to an external clocksignal.
 15. The memory module of claim 12 wherein the internal registersof the first and second memory device store the identification valuesafter a predetermined number of clock cycles of an external clocksignal.
 16. The memory module of claim 12 wherein the first memorydevice includes a reset register coupled to the reset signal line toreceive the identification value of the first memory device.
 17. Thememory module of claim 16 wherein the reset register is clockedsynchronously with respect to an external clock signal.
 18. The memorymodule of claim 16 wherein the identification register is coupled to thereset register to receive and store the identification value received bythe reset register at the first predetermined time.
 19. The memorymodule of claim 16 wherein the identification register is coupled to thereset register to receive and store the data in the reset register aftera predetermined number of clock cycles of the external clock signal. 20.The memory module of claim 12 wherein the second memory device includesa reset register coupled to the reset signal line at an output of thereset register of the first memory device.
 21. The memory module ofclaim 20 wherein the reset register of the second memory device isclocked synchronously with respect to the external clock signal.
 22. Thememory module of claim 21 wherein the identification register is coupledto the reset register to receive and store the data in the resetregister at the second predetermined time.
 23. The memory module ofclaim 22 wherein the first predetermined time and the secondpredetermined time are the same.
 24. A method of assigningidentification values to a plurality of memory devices disposed on amemory module, the plurality of memory devices include a first memorydevice having a programmable register for storing an identificationvalue and second memory device having a programmable register forstoring an identification value, wherein the memory module includes abus to electrically connect the first and the second memory devices,wherein the bus includes a plurality of signal lines including a resetsignal line which electrically connects the first and the second memorydevices, a clock signal line for receiving an external clock signal, anddata lines for receiving first and second identification values, themethod comprising:applying the first identification value on the datalines for a predetermined number of clock cycles of the external clocksignal; applying a reset signal to the first memory device wherein, inresponse to the reset signal received by the first memory device, theprogrammable register in the first memory device stores the firstidentification value; applying the second information value on the datalines for the predetermined number of clock cycles of the external clocksignal; and applying a reset signal to the second memory device wherein,in response to the reset signal received by the second memory device,the programmable register in the second memory device stores the secondidentification value.
 25. The method of claim 24 wherein:the firstidentification value is a unique identification value which identifiesthe first memory device from the plurality of memory devices, includingthe second memory device, on the bus; and the second identificationvalue is a unique identification value which identifies the secondmemory device from the plurality of memory devices, including the firstmemory device, on the bus.
 26. The method of claim 24 wherein the firstidentification value is a special identification value which identifiesthe first memory device and a plurality of memory devices, including thesecond memory device, on the bus.
 27. A method of assigningidentification values to first and second memory devices disposed on amemory module having a bus which electrically connects the first and thesecond memory devices, wherein the bus includes a plurality of signallines including a daisy-chained signal line which electrically connectsthe first and the second memory devices in a daisy-chained manner, themethod comprising:applying first and second identification values on thedaisy-chained signal line; storing the first identification value in aprogrammable register in the first memory device at a firstpredetermined time; and storing the second identification value in aprogrammable register in the second memory device at a secondpredetermined time.
 28. The method of claim 27 wherein:the firstidentification value is a unique identification value which identifiesthe first memory device from the plurality of memory devices on the bus;and the second identification value is a unique identification valuewhich identifies the second memory device from the plurality of memorydevices on the bus.
 29. The method of claim 27 wherein the firstidentification value is a special identification value which identifiesthe first memory device and a plurality of memory devices on the bus.